Acoustic structure with increased bandwidth suppression

ABSTRACT

The bandwidth or acoustical range of a nacelle or other type of acoustic structure is increased by acoustically coupling honeycomb cells together to form pairs of acoustic cells that have an effective acoustic or resonator length that is up to twice that of either acoustic cell taken alone.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to acoustic structures that areused to attenuate noise that emanates from a particular source. Moreparticularly, the present invention is directed to providing relativelythin acoustic structures that are capable of attenuating a wide range ofnoise frequencies including relatively low-frequency noise, such as thelow-frequency noise that is generated by the engines of aircraft.

2. Description of Related Art

It is widely recognized that the best way of dealing with excess noisegenerated by a specific source is to treat the noise at the source. Thisis typically accomplished by adding acoustic damping structures(acoustic treatments) to the structure of the noise source. Oneparticularly problematic noise source is the jet engine used on mostpassenger aircraft. Acoustic treatments are typically incorporated inthe engine inlet, nacelle and exhaust structures. These acoustictreatments include acoustic resonators that contain relatively thinacoustic materials or grids that have millions of holes that createacoustic impedance to the sound energy generated by the engine.

Honeycomb has been a popular material for use in aircraft and aerospacevehicles because it is relatively strong and lightweight. For acousticapplications, such as engine nacelles, acoustic materials are added tothe honeycomb structure so that the honeycomb cells are acousticallyclosed at the end located away from the engine and covered with a porouscovering at the end located closest to the engine. The closing of thehoneycomb cells with acoustic material in this manner creates anacoustic resonator that provides attenuation, dampening or suppressionof the noise. Acoustic septums are also usually located within theinterior of the honeycomb cells in order to provide the resonator withadditional noise attenuation properties.

A basic problem facing acoustic engineers is to make the nacelle as thinand lightweight as possible while still providing adequate suppressionor dampening of the sound wave frequencies over the entire range ofnoise generated by the jet engine. This basic design problem iscomplicated by the fact that the trend in newer models of large jetengines is to produce additional noise at lower frequencies. The newengine designs tend to use fewer fan blades that produce more by-passair at a slower velocities. This results in the production of enginenoise having a lower frequency.

The particular frequencies of noise that are dampened by a givenhoneycomb cell or resonator is directly related to the depth of thecell. In general, as the frequency of the noise decreases, the depth ofthe cell must be increased in order to provide adequate dampening orsuppression. Relatively thin nacelles having cell depths on the order of1 inch or less are adequate for absorbing the higher frequency rangesgenerated by a jet engine. However, in order to absorb the lowerfrequencies that are being generated by newer jet engines, acoustic cellor resonator depths on the order of 2½ inches or more are required.

One approach to solving the problem of absorbing the lower frequency jetnoise is to simply build nacelles with deeper cells. However, thisresults in an increase in the size and weight of the nacelle which iscontrary to the design goal of providing nacelles that are as thin andlight weight as possible. In addition, the increase in weight and sizeof the nacelle required to absorb low-frequency noise may beunacceptable, especially for larger aircraft engines where the size andweight of the nacelle is a major engineering design consideration.

There presently is a need to design engine nacelles and other acousticstructures where the acoustic structure is capable of suppressing awider range of noise frequencies without increasing the thickness orweight of the nacelle acoustic structure.

SUMMARY OF THE INVENTION

In accordance with the present invention, it was discovered that thebandwidth or acoustical range of a nacelle or other type of acousticstructure can be increased by acoustically coupling honeycomb cellstogether to form pairs of acoustic cells that have an effective acousticor resonator length that is up to twice that of either acoustic celltaken alone. This increase in effective resonator length produces anacelle or acoustic structure that is capable of absorbing relativelylow noise frequencies without increasing the thickness or weight of thenacelle.

The present invention is directed to acoustic structures, in general,and to nacelles for aircraft engines, in particular. The acousticstructures in accordance with the present invention include a honeycombthat has a first edge located closest to the noise source and a secondedge located away from the noise source. The honeycomb includes aplurality of first acoustic cells wherein each of the first acousticcells shares a common wall with a second acoustic cell. Each of thefirst acoustic cells is terminated by a first acoustic barrier that islocated at or near the second edge of the honeycomb. The second acousticcells are terminated by a second acoustic barrier that is also locatedat or near the second edge of the honeycomb.

As a feature of the present invention, an acoustic pathway is located inthe common wall between the first and second acoustic cells toacoustically couple the cells together. The acoustic pathway is locatedbetween the first edge of the honeycomb and the first and secondacoustic barriers. A third acoustic barrier is provided in the secondacoustic cell to provide an acoustic termination of the second acousticcell at the first edge of the honeycomb or between the first edge of thehoneycomb and the acoustic pathway. The coupling of acoustic cells andthe acoustic barriers provide a first noise attenuation zone thatincludes the first acoustic cell, as well as the portion of the secondacoustic cell located between the second acoustic barrier and the thirdacoustic barrier. As a result, the first noise attenuation zone orresonator has an effective acoustic length that can be up to two timesthe depth of the honeycomb.

As a further feature of the present invention, placement of the thirdacoustic barrier at a position between the first edge of the honeycomband the acoustic pathway provides for a second noise attenuation zonethat has a length equal to the distance between the third acousticbarrier and the first edge of the honeycomb. As a result, the secondnoise attenuation zone or resonator has an effective acoustic lengththat is shorter than the depth of the honeycomb.

A wide variety of effective acoustic lengths for the first and secondnoise attenuation zones can be achieved by simply varying the distancebetween the third acoustic barrier and the first edge of the honeycombfor a given pair of coupled acoustic cells. The present inventionprovides a significant advantage over conventional acoustic honeycombwhere the acoustic cells all have the same effective acoustic lengthsand the only way of lengthening the cells is to increase the thicknessof the honeycomb.

The acoustic coupling of cells together in accordance with the presentinvention provides noise attenuation zones that can have effectiveacoustic lengths which range from a fraction of the honeycomb thicknessup to twice the thickness of the honeycomb or more. The ability to formacoustic cells having lengths that are smaller or greater than thethickness of the honeycomb provides a significant increase in the bandwith or range of frequencies that can be absorbed by the acoustichoneycomb structure. In addition, the ability to acoustically lengthenhoneycomb cells without increasing the honeycomb thickness is especiallyuseful for jet engine nacelles where it is desirable to make thehoneycomb as thin as possible while still providing acoustic resonatorsthat are capable of dampening low-frequency jet engine noise.

The above described and many other features and attendant advantages ofthe present invention will become better understood by reference to thefollowing detailed description when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an exemplary acoustic structure in accordance with thepresent invention prior to the solid and the porous face sheets beingbonded to the acoustic honeycomb.

FIG. 2 shows a portion of a nacelle in place adjacent to a an enginenoise source.

FIG. 3 is a perspective view of a partial acoustic honeycomb showing theacoustic pathways between the cells.

FIG. 4 is a schematic view showing the acoustic properties of first andsecond noise attenuation zones that are formed by acoustically couplingtwo adjacent acoustic cells together.

FIG. 5 is a further schematic drawing showing the acoustic properties ofthe first and second noise attenuation zones.

FIG. 6 is a simplified view of an acoustic honeycomb showing anexemplary arrangement for acoustically coupling the honeycomb cellstogether in order to provide lower-frequency noise dampening

FIG. 7 is a simplified view of an acoustic honeycomb showing analternate exemplary configuration for coupling the honeycomb cellstogether which also provides for low-frequency noise dampening

FIG. 8 is another simplified view of an acoustic honeycomb showing afurther exemplary configuration for coupling the acoustic cells togetherin order to increase the low-frequency dampening capability of theacoustic structure without increasing the thickness of the structure.

DETAILED DESCRIPTION OF THE INVENTION

A partially exploded view of a portion of an exemplary acousticstructure 10 in accordance with the present invention is shown inFIG. 1. The acoustic structure 10 includes an acoustic honeycomb 12which is sandwiched between a porous face sheet 14 and a solid acousticbarrier face sheet 16. The assembled acoustic structure 10 is shown inFIG. 2 where it is located adjacent to a noise source 18 which isgenerating noise as represented by arrows 20. Although the acousticstructure of the present invention may be used for dampening noise froma wide variety of noise sources, the acoustic structure is particularlywell-suited for dampening noise generated by aircraft engines andparticularly the large engines used for commercial aircraft.Accordingly, the acoustic structure shown at 10 in FIG. 2 is typicallypart of a nacelle which surrounds the central core of a turbofan jetengine 18.

As shown in the FIGS. 1-3, the honeycomb 12 includes a first edge 22that is located closest to the noise source 18 and a second edge 24 thatis located away from the noise source 18. As a feature of the presentinvention, the honeycomb 12 includes cells 28 and 30 that are coupledtogether by way of an acoustic pathway 26 to form pairs ofacoustically-coupled cells. The individual pairs of cells 28 and 30share a common wall in which the acoustic pathway 26 is formed. Eachcell 28 may be viewed as a first acoustic cell that is defined by thehoneycomb walls that extend between the first and second edges 22 and24. Each cell 30 may be viewed as a second acoustic cell that is alsodefined by the honeycomb walls that extend between the first and secondedges 22 and 24. The solid face sheet 16 serves as a first acousticbarrier 32 for the first acoustic cells and a second acoustic barrier 34for the second acoustic cells.

Although it is preferred that the acoustic barriers 32 and 34 beprovided by a single solid face sheet located along the second edge 24of honeycomb 12, it is also possible to form the first and secondacoustic barriers 32 and 34 with individual solid inserts that aredisplaced within the honeycomb cells away from the honeycomb cell edge.The positioning of such individual solid inserts must be such that theacoustic pathway 26 is located between the first edge of the honeycomb22 and the first and second acoustic barriers 32 and 34.

The acoustic pathway can be located in the common cell wall at aposition spaced away from the honeycomb second edge. However, it ispreferred that the acoustic pathway be formed by arcuate shaped slots26, as shown in FIG. 3, which seat against the solid face sheet 16 toprovide a closed arched shaped acoustic pathway. The acoustic pathwaycan have a wide variety of shapes provided that the opening in thecommon cell wall is sufficiently large to allow sound waves to travelthrough the pathway from the first acoustic cell 28 to the secondacoustic cell 30. Arch shaped or other contoured openings of the typeshown at 26 are preferred since they reduce the chance of fatiguecracking of the cell wall.

As a further feature of the invention, a third acoustic barrier 36 isprovided in the second acoustic cells 30. The third acoustic barriers 36may be located along the first edge 22 of the honeycomb. However, it ispreferred that the third acoustic barriers 36 be formed by individualsolid inserts that are displaced inward from the first edge 22 of thehoneycomb. The displacement of the acoustic barrier 36 into the secondacoustic cell 30 provides additional sound dampening as will bediscussed below.

FIGS. 4 and 5 are schematic representations of the acoustic dampeningproperties that are achieved when the first and second acoustic cells 28and 30 are paired together by way of an acoustic pathway 26 inaccordance with the present invention. As shown in FIG. 4, noise 20enters the honeycomb through porous face sheet 14. The sound waves, asrepresented by arrow 21, travel down through the first acoustic cell 28until they reach the first acoustic barrier 36 where they are directedlaterally through the acoustic pathway 26. The second acoustic barrier34 prevents the sound waves from escaping so that they are directed backup the second acoustic cell 30 until they are stopped by the thirdacoustic barrier 36. The acoustically coupled-cells provide two noiseattenuation zones or resonators that are capable of dampening orimpeding noise having different wavelengths. The first noise attenuationzone is formed by the first acoustic cell 28 and that portion of thesecond acoustic cell 30 located below the third acoustic barrier 36. Theeffective acoustic or resonator length of the first noise attenuationzone is (h+h₁). The second noise attenuation zone is formed by thatportion of the second acoustic cell 30 that is located between the thirdacoustic barrier 36 and the first edge of the honeycomb. The effectiveresonator length of the second attenuation zone is (h−h₁).

Referring to FIG. 5, the two noise attenuation zones or resonators areshown schematically side-by-side where the first noise attenuation zoneis shown at 38 and the second noise attenuation zone is shown at 40. Theacoustic coupling of the two cells together forms one resonator 38 thatis substantially deeper than the other resonator 40. Accordingly,instead of having an acoustic structure where all cells are the same,the present invention provides the significant advantage of having onerelatively long or deep acoustic resonator that is capable of dampeningrelatively low noise frequencies, while at the same time providing asecond resonator that is capable of dampening noise frequencies thathave a relatively higher frequency.

Additional frequency dampening and attenuation can be provided byincluding one or more acoustic septums within one or both of the coupledacoustic cells. For example, acoustic septum 42 is included in the firstacoustic cell 28 to provide an attenuator with two degrees of freedom. Asecond acoustic septum 44 may optionally be included in the secondacoustic cell 30 to provide an attenuator with three degrees of freedom.

The acoustic septum can be made from any of the standard acousticmaterials used it to provide noise attenuation including woven fibersand perforated sheets. The use of the woven fiber acoustic septums ispreferred. These acoustic materials are typically provided as relativelythin sheets of an open mesh fabric that are specifically designed toprovide noise attenuation. It is preferred that the acoustic material bean open mesh fabric that is woven from monofilament fibers. The fibersmay be composed of glass, carbon, ceramic or polymers. Monofilamentpolymer fibers made from polyamide, polyester, polyethylenechlorotrifluoroethylene (ECTFE), ethylene tetrafluoroethylene (ETFE),polytetrafluoroethyloene (PTFE), polyphenylene sulfide (PPS),polyfluoroethylene propylene (FEP), polyether ether ketone (PEEK),polyamide 6 (Nylon 6, PA6) and polyamide 12 (Nylon 12, PA12) are just afew examples. Open mesh fabric made from PEEK is preferred for hightemperature applications, such as nacelles for jet engines. Exemplaryseptums are described in U.S. Pat. Nos. 7,434,659; 7,510,052 and7,854,298. Septums made by laser drilling plastic sheets or films mayalso be used.

There are a variety of ways in which adjoining acoustic cells can becoupled together to form the first and second noise attenuation zones.Some examples of possible cell coupling configurations are shown inFIGS. 6, 7 and 8. In these figures, cells that are numbered 1 correspondto first acoustic cells 28 and cells that are numbered 2 correspond tosecond acoustic cells 30. The acoustic pathways connecting the two cellstogether are shown as solid bars 3.

If desired, all of the cells of a given acoustic structure may beacoustically coupled to form acoustic pairs or acoustically coupled cellpairs can be dispersed among non-acoustically coupled cells. In somesituations, it is desirable that only certain portions of the acousticstructure include acoustically coupled cell pairs. For example, in manyacoustic structures, including nacelles, it is common practice toinclude openings in the honeycomb cells that are located in the lowerportions of the structure in order to allow any accumulated water todrain from the structure. The drainage openings interconnect a largenumber of honeycomb cells together in order to ensure that adequatewater drainage pathways are provided to drain all of the water from thestructure. Use of such widely interconnected water drain openings isinconsistent with the present invention wherein the effective length ofan acoustic cell is increased by way of coupling of two acoustic cellstogether.

The present invention has been described with respect to the acousticcoupling of only two adjacent cells together. If desired, three or moreacoustic cells could be acoustically linked together using acousticpathways and acoustic barriers in the same manner as described abovewith respect to the acoustic coupling of two cells. The linking of morethan two acoustic cells together is warranted in those situations wherethe honeycomb is relatively thin and/or a relatively long resonator isrequired in order to dampen very low frequency noise. The number of thecells that are linked together would be determined by a combinedconsideration of the desired honeycomb thickness and the frequency rangeover which attenuation or dampening is desired.

The present invention has focused on the coupling of two cells togetherbecause the size and noise frequency requirements for dampening jetengine noise can be met using nacelles in which the honeycomb structureincludes the coupling of two cells. For example, the low-end frequencyrange produced by large commercial jet engines is in the range of 500 to2000 Hz. It was found that honeycomb having a thickness of around 1 to 2inches does not have the capability of dampening such low-frequencynoise. However, by acoustically coupling the cells together, effectiveresonator lengths can be obtained that are able to suppress suchlow-frequency engine noise.

The materials used to make the honeycomb can be any of those typicallyused in acoustic structures including metals, ceramics and compositematerials. Exemplary metals include aluminum and aluminum alloys.Exemplary composite materials include fiberglass, Nomex and variouscombinations of graphite or ceramic fibers with suitable matrix resins.Matrix resins that can withstand relatively high temperatures (300° F.to 400° F.) are preferred . The materials used to make the solid facesheet 16 can also be any of the solid face sheet materials commonly usedfor acoustic structures which typically include the same type ofmaterials used to make the honeycomb structure. The materials used tomake the porous face sheet 14 can also be any of the materials commonlyused for such porous structures provided that the pores or perforationsin the structure are sufficient to allow the sound waves from the jetengine or other source to enter into the acoustic cells or resonators.

In general, the honeycomb cells will typically have a cross-sectionalarea ranging from 0.05 square inch to 1 square inch or more. The depthof the cells (honeycomb thickness or core thickness) will generallyrange from 0.25 to 3 inches or more. For jet engine nacelles, thehoneycomb cells will typically have a cross-sectional area of betweenabout 0.1 to 0.5 square inch and a depth of between about 1.0 and 2.0inches. As an exemplary advantage of the present invention, nacelleshaving honeycomb cell depths at the lower end of the thickness range(1.0 inch) can provide the same low-frequency noise attenuation orsuppression that is provided by nacelles having thicknesses at the upperend of the thickness range (2.0 inch). For example, if the first andsecond acoustic barriers 32 and 34 are located at the second edge 24 ofthe honeycomb and the third acoustic barrier 36 is placed 0.25 inch intothe second acoustic cell 30 of an acoustic pair of cells 28 and 30, theresulting effective length of the acoustic cell pair is 1.75 inch and0.25 inch.

The ability to take a nacelle that is a certain thickness and increasethe effective resonator length up to two times and more is a significantadvantage, since it allows one to make the nacelle as thin andlightweight as possible, while still being able to dampen the relativelylower frequency noise that is being generated by your jet enginedesigns. In addition, the portion 40 of the second acoustic cell of thecell pair that is not used to extend the effective acoustic length ofthe first acoustic cell provides additional noise attenuation at adifferent (higher) frequency. This provides an increase in the range offrequencies (bandwidth) that can be effectively suppressed by theacoustic structure.

As mentioned previously, it is preferred that a solid face sheet 16 beused to close off the second edge of the honeycomb. In this situation,the first and second sound barriers 32 and 34 are all located along thesecond edge of the honeycomb. In accordance with the present invention,it is possible to increase the bandwidth or range of frequencies thatcan be suppressed by simply varying the location of the third acousticbarrier 36 within the second acoustic cell 30 of each acoustic pair 28and 30. Even further increases in bandwidth suppression can be obtainedby acoustically linking three or more cells together and combining theseacoustically linked cells with the acoustically coupled cells atselected locations throughout the acoustic structure. Of course, it isalso possible to provide acoustic structures which include singleacoustic cells, acoustically coupled cell pairs and acoustically linkedcell triplets.

Further variations in noise attenuation can be achieved by varying thesize of the acoustic pathway 26 between coupled acoustic cell pairs 28and 30. The size of the acoustic pathway is chosen based on theeffective length of the resonator (first noise attenuation zone) thatresults from the acoustic pathway between coupled cells and thefrequency of noise that is being suppressed.

Having thus described exemplary embodiments of the present invention, itshould be noted by those skilled in the art that the within disclosuresare exemplary only and that various other alternatives, adaptations andmodifications may be made within the scope of the present invention.Accordingly, the present invention is not limited by the above-describedembodiments, but is only limited by the following claims.

1. An acoustic structure for reducing noise generated from a source,said acoustic structure comprising: a honeycomb comprising a first edgelocated closest to said source and a second edge, said honeycombcomprising a first acoustic cell defined by a plurality of walls thatextend between said first and second edges and a second acoustic celldefined by a plurality of walls that also extend between said first andsecond edges of said honeycomb wherein said first acoustic cell and saidsecond acoustic cell share a common wall: a first acoustic barrier forsaid first acoustic cell; a second acoustic barrier for said secondacoustic cell; a surface in said common wall that defines an acousticpathway between said first acoustic cell and said second acoustic cell,wherein said acoustic pathway is located between the first edge of saidhoneycomb and said first and second acoustic barriers; and a thirdacoustic barrier for said second acoustic cell wherein said acousticpathway is located between the second acoustic barrier and said thirdacoustic barrier to thereby provide a first noise attenuation zonecomprising the portion of said first acoustic cell located between saidfirst edge and said first acoustic barrier and the portion of saidsecond acoustic cell located between said second acoustic barrier andsaid third acoustic barrier, wherein said acoustic pathway is the onlyopening into the portion of said first noise attenuation zone thatcomprises said portion of said second acoustic cell located between saidsecond acoustic barrier and said third acoustic barrier.
 2. An acousticstructure according to claim 1 wherein said third acoustic barrier isdisplaced away from the first edge of said honeycomb such that saidthird acoustic barrier and the portion of said second acoustic celllocated between said third acoustic barrier and said first edge form asecond noise attenuation zone.
 3. (canceled)
 4. (canceled)
 5. Anacoustic structure according to claim I wherein said first acousticbarrier and said second acoustic barrier are located at the second edgeof said honeycomb.
 6. An acoustic structure according to claim 1 whereinan acoustically porous sheet covers the first edge of said honeycomb. 7.An acoustic structure according to claim 1 wherein at least one acousticseptum is located within said first acoustic cell.
 8. An acousticstructure according to claim 1 wherein at least one acoustic septum islocated within said second acoustic cell.
 9. An acoustic structureaccording to claim 1 wherein said acoustic structure is a nacelle for anengine.
 10. An airplane comprising a nacelle according to claim
 9. 11. Amethod for making an acoustic structure for reducing noise generatedfrom a source, said method comprising the steps of: providing ahoneycomb comprising a first edge located closest to said source and asecond edge, said honeycomb comprising a first acoustic cell defined bya plurality of walls that extend between said first and second edges anda second acoustic cell defined by a plurality of walls that also extendbetween said first and second edges of said honeycomb wherein said firstacoustic cell and said second acoustic cell share a common wall;providing a first acoustic barrier for said first acoustic cell;providing a second acoustic barrier for said second acoustic cell;forming a surface in said common wall that defines an acoustic pathwaybetween said first acoustic cell and said second acoustic cell, whereinsaid acoustic pathway is located between the first edge of saidhoneycomb and said first and second acoustic barriers; and providing athird acoustic barrier for said second acoustic cell wherein saidacoustic pathway is located between the second acoustic barrier and saidthird acoustic barrier to thereby provide a first noise attenuation zonecomprising the portion of said first acoustic cell located between saidfirst edge and said first acoustic barrier and the portion of saidsecond acoustic cell located between said second acoustic barrier andsaid third acoustic barrier, wherein said acoustic pathway is the onlyopening into the portion of said first noise attenuation zone thatcomprises said portion of said second acoustic cell located between saidsecond acoustic barrier and said third acoustic barrier.
 12. A methodfor making an acoustic structure according to claim 11 wherein saidthird acoustic barrier is displaced away from the first edge of saidhoneycomb such that said third acoustic barrier and the portion of saidsecond acoustic cell located between said third acoustic barrier andsaid first edge form a second noise attenuation zone.
 13. (canceled) 14.A method for making an acoustic structure according to claim 11 whichincludes the step of locating at least one acoustic septum within saidfirst acoustic cell.
 15. A method for making an acoustic structureaccording to claim 11 which includes the step of locating at least oneacoustic septum within said second acoustic cell.
 16. A method formaking an acoustic structure according to claim 11 wherein said acousticstructure is a nacelle for an engine.
 17. A method for reducing thenoise generated from a source of noise, said method comprising the stepof at least partially surrounding said source of noise with an acousticstructure according to claim
 1. 18. A method for reducing the noisegenerated from a source of noise according to claim 17 wherein saidsource of noise is an engine and said acoustic structure is a nacelle.19. (canceled)
 20. (canceled)
 21. An acoustic structure according toclaim 7 wherein at least one acoustic septum is located within saidsecond acoustic cell.
 22. A method for making an acoustic structureaccording to claim 14 which includes the step of locating at least oneacoustic septum within said second acoustic cell.